1. Field of the Invention
The present invention relates to a vibration detection apparatus, a hologram apparatus, a vibration detection method for the vibration detection apparatus and a recording method for the hologram apparatus.
2. Description of the Related Art
Among hologram recording media adapted to record digital data as holograms is a photosensitive resin (e.g. photopolymer) sealed between substrates. To record digital data on a hologram recording medium as a hologram, a coherent laser beam from a laser device is first split into two beams by a PBS (Polarization Beam Splitter). Then, one of the two beams (hereinafter referred to as “reference beam”) and a laser beam (hereinafter referred to as “data beam”) produced by the other beam irradiating an SLM (Spatial Light Modulator) having digital data in the form of a two-dimensional contrast image pattern, which beam reflects information of the two-dimensional contrast image pattern, are irradiated into a hologram recording medium at a given angle. This allows the recording of the digital data into the hologram recording medium.
More specifically, the photosensitive resin making up the hologram recording medium has a finite number of monomers. When the laser beam (hereinafter referred to as “laser beam”) made up of the reference and data beams is irradiated thereinto, the monomers change into polymers correspondingly with the energy determined by the light intensity of the laser beam and the irradiation time. As a result of the transformation of the monomers into polymers, an interference fringe, made up of polymers, is formed correspondingly with the laser beam energy. Therefore, as a result of the formation of such interference fringes in the hologram recording medium, digital data is recorded as a hologram. Later, remaining monomers migrate (diffuse) to those locations that have consumed monomers. Further, by applying the laser beams again, changes of such monomers into polymers are iterated. It is to be noted that FIG. 2 schematically illustrates how monomers transform into polymers correspondingly with the laser beam energy in the hologram recording medium.
It is also to be noted that if a large amount of digital data must be recorded in the hologram recording medium, the incidence angle of the reference beam into the hologram recording medium may be varied to achieve a so-called “angle-multiplexed recording” to form a number of holograms. For example, a hologram formed in the hologram recording medium is called a page, whereas a multiplexed hologram made up of a number of pages is called a book. FIG. 3 illustrates relationship between a book and pages in the angle-multiplexed recording. This is a logical construct and not physically how the holograms are stored. As shown in FIG. 3, the incidence angle of the reference beam is varied to form, e.g., ten pages of hologram for a single book in the angle-multiplexed recording. Thus, the angle-multiplexed recording allows for the recording of a large amount of digital data.
To reproduce digital data from the hologram recording medium, on the other hand, the reference beam is irradiated into the interference fringes representing the digital data at the same incidence angle as when the interference fringes were formed. The reference beam diffracted by the interference fringes (hereinafter referred to as “reproduction beam”) is received by an image sensor or other means. The reproduction beam received by the image sensor or other means forms a two-dimensional contrast image pattern representing the above-mentioned digital data. Then, the digital data can be reproduced by demodulating this two-dimensional contrast image pattern by a decoder or other means.
Thus, when digital data is reproduced from a hologram recording medium, a two-dimensional contrast image pattern is reproduced from the reproduction beam. Therefore, the reproduction beam at an image sensor or other means must have a light intensity equal to or above a given level to allow the reproduction of the two-dimensional contrast image pattern. To provide the reproduction beam with at least the given level of light intensity, therefore, the interference fringes diffracting the reference beam must have a specified or higher value of diffraction efficiency, which represents the ratio of the reproduction beam light intensity to that of the incident reference beam. It is to be noted that the specified value of diffraction efficiency refers to the reproduction beam having the given level of light intensity.
In the multiplexed recording to a hologram recording medium, a number of holograms, that is, a number of interference fringes are formed as described above. For this reason, the plurality of interference fringes representing a plurality of holograms are formed with extremely small spacing therebetween. For example, assuming that the laser beam wavelength is λ, the incidence angle of the laser beam into the hologram recording medium θr and the refraction angle of the laser beam from the hologram recording medium θs, then the spacing between the interference fringes can be expressed by λ/|2 sin((θr−θs)/2)|. It is clear therefrom that the spacing between the interference fringes is a minimum of λ/2 when the angle is maximal (i.e., at 2 sin((θr−θs)/2). It is also clear that when a helium-neon laser with a wavelength λ of 633 nm is used, for example, as the laser beam, the minimum spacing between the interference fringes is 316.5 nm, an extremely small spacing. In the angle-multiplexed recording, therefore, a number of interference fringes must be formed with extremely precise spacing between them. This requires an accurate irradiation of the laser beam into the hologram recording medium.
Meanwhile, the interference fringes are formed in the hologram recording medium as a result of the transformation of a finite number of monomers into polymers. For this reason, each time a page, namely, a group of interference fringes are formed in the hologram recording medium during the angle-multiplexed recording, the monomers in the hologram recording medium gradually diminish in number. In the angle-multiplexed recording to the hologram recording medium, for this reason, the method has been employed where the irradiation time of the laser beam into the hologram recording medium is lengthened gradually to form a number of interference fringes having a diffraction efficiency equal to or greater than the above specified value. This well known 30 year old technique is called scheduling and for an example of scheduling in photopolymers. See Allen Pu, Kevin Curtis, and Demetri Psaltis, “Exposure Schedule for Multiplexing Holograms in Photopolymer Films,” Optical Eng., 35 (10):2824-2829 (1996).
However, such lengthening of the laser beam irradiation time means a higher likelihood of the hologram recording medium, the laser device and others being affected by external vibrations if the system operable to record and reproduce a hologram and the hologram recording medium undergo such vibrations. For this reason, if affected by vibrations of the given level or higher during the laser beam irradiation, the laser beam may fail to be accurately irradiated, so that no interference fringe is formed, or if affected by vibrations over a long period of time during the laser beam irradiation, interference fringes may not be formed with the very precise spacing therebetween as described above. As a result, digital data may fail to be accurately recorded in the hologram recording medium, so that the medium fails to present the full extent of its inherent capability to record a large amount of digital data.